const fp = require("lodash/fp");

// support.js
class Container {
  static of(value) {
    return new Container(value);
  }
  constructor(value) {
    this._value = value;
  }
  map(fn) {
    return Container.of(fn(this._value));
  }
}

class Maybe {
  static of(x) {
    return new Maybe(x);
  }
  isNothing() {
    return this._value === null || this._value === undefined;
  }
  constructor(x) {
    this._value = x;
  }
  map(fn) {
    return this.isNothing() ? this : Maybe.of(fn(this._value));
  }
}

// 练习1：使用 fp.add(x, y) 和 fp.map(f,x) 创建一个能让 functor 里的值增加的函数 ex1
let maybe = Maybe.of([5, 6, 1]);
let ex1 = maybe.map((i) => fp.map(fp.add(1), i));
console.log(ex1); // [6, 7, 2]

// 练习2：实现一个函数 ex2，能够使用 fp.first 获取列表的第一个元素
let xs = Container.of(["do", "ray", "me", "fa", "so", "la", "ti", "do"]);
let ex2 = xs.map((i) => fp.first(i));
console.log(ex2);

// 练习3：实现一个函数 ex3，使用 safeProp 和 fp.first 找到 user 的名字的首字母
let safeProp = fp.curry(function (x, o) {
  return Maybe.of(o[x]);
});
let user = { id: 2, name: "Albert" };
let ex3 = safeProp("name", user).map(fp.first);
console.log(ex3);

// 使用 Maybe 重写 ex4，不要有 if 语句
let ex4 = function (n) {
  if (n) {
    return parseInt(n);
  }
};

ex4 = (n) => Maybe.of(n).map(parseInt);
console.log(ex4(1), ex4());

module.exports = { Maybe, Container };
